According to the rapid spread of a broadband access for handling a large volume of data, communication traffic tends to increase year after year. Innovations in optical transmission techniques have been continuing in order to cope with the rapidly increasing communication traffic. In recent years, as a transmission technique for realizing a transmission capacity exceeding 100 Gbit/s per one wavelength, a digital coherent optical transmission technique has been developed (see, for example, NPL 1).
An optical transmission equipment using the digital coherent optical transmission technique includes a LAN interface unit that inputs and outputs a client signal of, for example, 100 GbE (Gigabit Ethernet) (Ethernet is a registered trademark), a WAN interface unit that is connected to an opposed optical transmission equipment and inputs and outputs an OTN (Optical Transport Network) signal, and a digital function unit. The digital function unit includes a client-signal processing LSI that performs termination processing of a client signal, an OTN framer LSI that transparently stores the client signal in an OTN frame, and a digital coherent DSP-LSI.
In the conventional coherent optical communication scheme, a local oscillation light source (LO) is provided in a receiver, a beat signal with a received optical signal is converted into a baseband or an intermediate frequency band, and a received equalized waveform is regenerated. This configuration enables high sensitive detection and static chromatic dispersion compensation (delay equalization) of an optical fiber, which is a transmission line, and the like. However, in the conventional coherent optical communication system, synchronization of frequencies/phases between a transmitted optical signal and a local oscillation light, polarization tracking, and the like have been significant technical problems. In the digital coherent optical transmission technique, such processing is realized by digital signal processing using a digital coherent DSP-LSI. Besides such processing, the digital coherent DSP-LSI performs CD (chromatic dispersion) compensation and PMD (polarization mode dispersion) compensation by a digital filter, demodulation processing of a modulated signal such as QPSK (Quadrature Phase Shift Keying) and QAM (Quadrature Amplitude Modulation), FEC (forward error correction) processing, and the like. In this way, in the digital coherent DSP-LSI, various functions necessary for performing coherent detection of a received optical signal are implemented.